1. Field of the Invention
This invention relates to the stabilization of vinylidene chloride interpolymers against light-induced degradation. More particularly, this invention is related to the stabilization of fire-resistant non-woven fabrics, including an interpolymer of vinylidene chloride and a mat of fibers, against actinic degradation and discoloration, i.e. yellowing.
2. Description of the Art
The fire resistance of certain polymers is directly related to the amount of chlorine contained therein; therefore, interpolymers containing a high amount of vinylchloride, in interpolymerized form, are used in applications requiring fire resistance. For example, fire-resistant coatings are formulated with vinylchloride interpolymers, wherein the amount of vinylchloride comonomer comprises at least about 50 percent, by weight, in interpolymerized form.
Vinylidene chloride contains almost 30 percent, by weight, more chlorine than vinyl chloride on an equivalent molar basis; therefore it is desirable to utilize vinylidene-chloride interpolymers in fire-resistant formulations. Unfortunately, when exposed to sunlight or ultraviolet radiation, vinylidene chloride interpolymers become brittle, discolor, crack, and lose tensile strength. In addition, about 5 percent of the total weight of powders of polyvinylidene chloride is evolved as hydrochloric acid upon exposure to ultraviolet radiation--all of which severely limits the utility of this material.
Commercial suppliers of polymeric materials have approached the problem of light-induced degradation of polymeric materials by the addition of compounds called "ultraviolet absorbers" which prevent ultraviolet radiation from penetrating the material. The most commercially significant class of ultraviolet absorbers are derivatives of 2-hydroxybenzophenone, phenyl esters, and cinnamic acid derivatives.
Another approach to the inhibition or prevention of light-induced degradation of interpolymers of vinylidene chloride has been by the addition of compounds referred to as "excited state quenchers." When interpolymers of vinylidene chloride are exposed to actinic energy, they are excited to levels of greater energy. Excited state quenchers interact with these energetically excited interpolymers so as to lower their excitation energy thereby avoiding or lessening degradation of the interpolymers. Useful excited state quenchers are 1,3 cyclooctadiene, 2(2'-hydroxy-5'-methylphenylbenzotriazol), and 2-hydroxy-4-methoxybenzophenone.
Unfortunately, even generally excellent stabilizing compounds are not entirely free of disadvantage; for example, only a limited amount of stabilizer can be added to inhibit or prevent light-induced degradation since above a low concentration of stabilizer there often is no proportional increase in protective activity as concentration increases. In addition, at high concentrations some stabilizers may even increase degradation and often increased levels of ultraviolet stabilizers have a deleterious effect on the desired physical and chemical properties of the polymeric materials to be stabilized. Further still, stabilizing compounds, being of low molecular weight in comparison to the polymer molecules to be stabilized, are often removed from the polymeric material through exudation, volatilization, leaching, or other processes, thereby reducing the effective life of the stabilizer in the polymeric material and increasing the possibility of unwanted volatilization of toxic chemical species.
The need remains to inhibit or prevent the light-induced degradation of polymeric materials derived from vinylidene chloride. The satisfaction of this need would preferably preserve or enhance the mechanical properties of the interpolymers of vinylidene chloride while not adding a toxic substance which may exude, volatilize, or be leached out of the material during use. Moreover, coatings that yellow when exposed to sunlight have limited decorative use.
Many comonomers have been interpolymerized with vinylidene chloride to modify the nature thereof. For example, in U.S. Pat. No. 3,922,451, it is taught that adhesion promoting comonomers such as vinylcarboxylic acids, vinylcarboxylic acid amides, and hydroxy containing vinyl monomers may be interpolymerized with vinylidene chloride to provide latexes useful as beverage container coatings. The latexes may include conventional thickeners, surfactants and wetting agents, but are essentially free from any solids other than the interpolymer. Moreover, it is disclosed that other comonomers may be present in the vinylidene chloride interpolymer including acrylonitrile, methacrylonitrile, alkylacrylates, alkacrylates, vinylmonomers, etc. The other comonomers are likely included to disrupt the crystallinity of the vinylidene chloride (although the reason for the inclusion of such other comonomers is not specifically set forth) and thereby provide interpolymers which are film-forming at the temperature used in beverage container coating. It is clear, however, that the patentees are not concerned with light-induced degradation or discoloration of the vinylidene chloride interpolymers of the patent since the coatings are applied to the interior of the beverage container.
In view of the foregoing, one object of this invention is to inhibit or prevent light-induced degradation of the interpolymers of vinylidene chloride.
Another object of this invention is to provide fire-resistant, light-stable non-woven fabrics including a vinylidene chloride interpolymer as a bonding material.
These and other objects and advantages of the invention will become apparent in view of the following description of the invention.